WeightWatchers bathroom scales come with misleading claims re their accuracy, even their precision..

Caveat: In what follows I shall be referring to kilograms  weight when, properly speaking, at least on a scientific site, it should of course be kilograms mass. Yes, this blogger is aware of the distinction between mass and weight. But the topic today is what in ordinary everyday life is referred to as one's body weight, changes in which we increasingly may wish to be aware of both precisely and accurately (more later on the difference between precision and accuracy). For any diehards who might be offended by my constant references to kilograms weight, then the advice is simple. Mentally substitute mass for weight OR if numerically inclined, multiply all kilogram figures by g, the acceleration due to Earth's gravity, whose approximate average value on Planet Earth is 9.8 metres/sec/sec, thus converting kilograms mass to the SI unit of force, measured in newtons, recalling that weight is a force.

So where does the WeightWatchers organization come in ( against whom I have no axe to grind except for a recent purchase of their bathroom scales that I now regret)?  Yes. I'm sure many folk have reason to be grateful to WW for their weight loss classes. Here's the object of my current aversion to WeightWatchers:

Sleek slimline WeightWatchers bathroom scales, with that digital readout.

They come with this booklet, clearly written by a Less Numerate. Less Scientifically-Aware member of their organization .

Title of the enclosed 'book of words' describes the supplied goods as a "precision scale"?  Really? Do the scales report the same answer on a series of repetitive weighings (regardless of whether the answer is accurate or not)?

"Precision" in the WeightWatchers dictionary would appear to refer to the readout, displaying weight to the nearest 0.1 kilogram. The calibration of the scale and readout has absolutely nothing to say regarding precision or accuracy, needless to say.  Indeed, a reading to the nearest 0.1kilogram conveys an entirely SPURIOUS degree of precision and accuracy, as is clear simply from reading the accompanying book of words. 

Again, it  claims it's a "precise" measuring instrument.  Really?  

It says it is accurate too (provided you stand still AND try to stand on the same area of the scales' platform). Can hedged-around "accurate" really be "accurate"? Oh, and it says not to "get on an off the scale repeatedly".  So how is one supposed to check the scales for precision, i.e. repeatability?

Deeper into the book of words we find this:

"Even a slight change in your position will cause a different reading"

Er, how can these scales be precise, never mind accurate, if a slight change in position gives different readings?  Weight is supposed to be directly proportional to mass, and directly proportional to gravitational attraction. It's not supposed to be affected by where one stands on a small platform. Nor is it supposed to be affected by the precise location of the scales on one's bathroom floor-covering (mine being vinyl plus underlay on top of concrete).

I guess it's that final sentence that is the give-away: This scale is designed for home use only and is not suitable for professional purposes".

So why the preamble that trumpets the scales' precision and accuracy? Do WeightWatchers know the difference between those two measures?

Scales that were inaccurate, but reasonably precise. might be tolerable, providing the inaccuracy were not excessive, say within 1% of the "right" answer, determined by some independent measure of greater intrinsic accuracy. Then one could take repetitive readings, hopefully not too dissimilar, and take, say, an average of 5.  But it's a waste of time doing that. The scales are NOT precise, and it's not difficult to see why, given that readings are affected not just by where one places the scales on one's bathroom floor, but where one stands on the platform.

So what was it that first alerted this science blogger to the lack of precision and accuracy in these scales (or this type of scales?).

Answer: it was first finding that dietary and exercise regimes that lasted a week were frequently failing to record loss of weight. Indeed, sometimes the answer could be as much as a kilogram greater than intital reading, which is dispiriting to say the least.

Note that I have been recording week-to-week changes, as advised in the book of words, so as not to be unrealistic re expectations.

The scales may be "valuable for monitoring your weight over months", but I question whether they are fit-for-purpose in measuring changes from one week to the next.

Just in case you had forgotten... "Your WeightWatchers scale is a precise measuring instrument..."

Oh no it's not!

"To ensure accurate readings, always try to stand on the same area of the scale platform, and DO NOT MOVE."

Well. I hate to be the bearer of bad news, WeightWatchers. but scales that give different answers depending on precisely where one stands. or which are highly sensitive to the slightest movement simply cannot be ACCURATE. And it goes without saying that they will not be PRECISE either.

It mightn't be so bad, except for the fact that scales for which I paid some £25 as I recall, influenced in no small measure by the WeightWatchers label were subsequently spotted elsewhere (or a near-identical looking model) at half the price. I have paid a hefty premium for the name. What a pity that WeightWatchers could not have been more forthcoming re the limitations of the product, both as regards the product name (dropping the term "precision) and those entirely misleading references to precision and accuracy in that book of words.

I have decided reluctantly to go back to my old scales.  OK, so it was difficult to read the scale despite the inbuilt glass lens, so one had to be content with knowing one's weight to within approximately the nearest kg or so. But that can spare one a lot of disappointment after a week or so of dietary restriction and exercise.

I would urge WeightWatchers to alter the display, either to give weight rounded up or down to the nearest kg, OR, if continuing to display to 0.1kg, to follow that with a  suitable and realistic +/- confidence range.Alternatively. tell one where one can buy 50kg calibrated masses, say, that one would place on the scales first, in order to determine if the scales are reading 'high' or 'low' on a particular day.

Conclusions. "WeightWatchers precision bathroom scales". No, something that cannot be relied upon to produce the same answer when one steps off, and then back on again, cannot be described as a precision instrument. Call it WeightWatchers Convenience bathroom scales, or WeightWatchers Handy Bathroom scales. Don't describe them as precision bathroom scales. It's doubtful whether any simple gadget that attempts to measure the force of the Earth's gravitational pull on an object, no matter how stationary the latter, especially of a live 'object' like a living breathing human being, can ever hope to be "precise". In fact, the WeightWatchers scales are imprecise - something that the accompanying book of words essentially acknowledges.

Afterthought: On a lighter note, maybe someone, somewhere, sometime will invent 'bathroom'(?) scales that genuinely display body mass in kilograms. Then none of the above gripes about  deficient mechanisms that measure mass as downward force on a dodgy platform would be necessary. Such a mass balance would be ideal in a world of interplanetary travel, or, come hyperdrive propulsion and warp speeds, interstellar travel to more inviting destinations too. So far we have CERN's LHC as a teaser. LHC = Large Hadron Collider. Hadrons are the massive particles - protons and neutrons - in the atomic nucleus that are responsible for most of atomic mass, and thus body mass too. So what's needed to determine body mass is an LHS - a Large Hadron Scanner. Sadly and realistically, the initial late 21st century (?) prototypes might tend to resemble CT scanners in hospitals. in which the human-to-be-scanned is slid on casters into a tunnel. So maybe it's too much to expect compact, space-efficient mass-measuring bathroom scales any time soon. Mass-measuring garage scales maybe, the latter cleared of all the other clutter like lawnmowers, hosepipes, algae-covered garden furniture  etc, maybe, just maybe.

Update 8th May 2014: have just this morning discovered the source of the SYSTEMATIC error on my bathroom scales. If one stands leaning slightly forward (as one does to read one's weight on the LCD), with one's weight mainly acting through the balls of the feet, one gets a reading that is typically 1kg more than if standing more upright, with weight mainly on the heels.

What's more, there's often a series of creaking sounds from the scales when one steps off. The error and the creaks are due no doubt to the weight being measured at 4 different points, i.e. the corner rubber feet on which the balance stands.

Whatever the mechanism, it is clearly UNFIT FOR PURPOSE, especially when it carries the name "Weight Watchers", where it may take at least a week (typically two on my gentle weight loss regime) to lose a single kilogram.

You have put your name to imported rubbish, Weight Watchers that probably costs next to nothing to mass-produce. Shame on you.


Update (intended for my previous lithium battery posting, but never mind): The BBC is in fact doing a series of features on the applications of the chemical elements and their compounds in everyday life. It's done in their "Magazine" rather than under a science/technology tab (interesting), and I see there have been previous ones in the series that I have missed (e.g. on aluminium) which I may return to at a later date. 

The reason for this update is that a new one has just appeared on chlorine ("From Toxic Chemical to Household Cleaner").

It's quite well done, but there's still a little confusion between sodium the element, comprising sodium atoms, and sodium in its compounds like common salt (sodium chloride), caustic soda (sodium hydroxide) etc where the sodium id present as the stable Na+ ion (a sodium atom stripped of its outermost electron).

Yes, the article begins by describing the heavy demands that the chlor-alkali process makes on electrical energy.  But it then attempts to illustrate that with reference to the electrolysis of (molten) sodium chloride, to produce metallic sodium (atoms) at the negative electrode and chlorine gas at the positive electrode. Note the complete absence of water (the system requires high temperatures at which sodium chloride is molten).  But that is not the is not the chlor-alkali process, which uses brine (a strong solution of common salt in water).The end product from the aqueous cell is not sodium metal (noting below the 'grey area' if it's an obsolescent Caster-Kellner cell producing sodium amalgam) but sodium hydroxide solution, along with hydrogen and chlorine gas. The sodium in sodium hydroxide is still Na+, so there has been no nett change in the state of the sodium (with a correspondingly smaller electrical energy requirement compared with the chlor-alkali process).

Overall reaction:

2NaCl  +   2H2O  =  2NaOH  +  H2  +  Cl2   (apologies for the = sign; arrow heads don't work)  


In ionic terms, the two half-reactions are as follows:

(1)      2H+  +  2 electrons at the negative cathode  =   H2

i.e. 2 hydrogen ions from 2 molecules of water are reduced by electrons to give a diatomic molecule of hydrogen gas.


(2)  2Cl-    = Cl2  +  2 electrons

i.e. 2 chloride ions are oxidised to a diatomic molecule of chlorine gas, returning the two  "borrowed" electrons from the electrical power source.



Geekish footnote:  However, some misunderstanding can be excused: the article makes brief reference to the old (19th century) process used in the chlor-alkali industry using mercury as the negatively-charged electrode (now largely abandoned due to environmental pollution by toxic mercury). In that so-called Castner-Kellner process, the sodium ions ARE discharged as sodium atoms into the mercury, where they form a liquid amalgam (with properties somewhat intermediate between those of an alloy mixture and inter-metallic compound(s) of sodium and mercury, NaxHgy etc). The amalgam is then run off and reacted with water to produce sodium hydroxide and hydrogen, so overall the process is the same as any other chlor-alkali electrolytic cell, with a form of elemental sodium ("protected" initially from water by its mercury keeper) being a temporary feature.

Further reading from "How Stuff Works": Inside a bathroom scale


The mechanical setup corresponds with what one was taught in elementary physics as that of a Class 3 lever, operating at what might be termed a mechanical disadvantage (example: tweezers). . The example given shows how a 112 kg mass is in fact measured at one tenth that figure (through having the fulcrum at a far end of the lever, the load (spring and pointer) at the opposite end, and the effort (the person's weight) acting close to the fulcrum.